Star delta motor starter
Star delta motor connection
For star delta starter, the motor connection must have 6 cables from control panel and 6 terminals at induction motor ( U1,U2,V1,V2,W1,W3).To wiring the motor connection for star delta starter, the important thing that we must fully understand is about the basic of STAR DELTA MAGIC TRIANGLE.
For detail about star delta starter, please read about it. From this triangle diagram, we can determine the correct phase, cable termination for right terminal and the rotation. As we know, the star delta starter is so tricky if we not fully understand the concept and their method. This time I want share my technique when perform wiring task and connected to the star delta starter for induction motor.
Don`t worry, it’s simple and easy if we understand the basic concepts. I explains detail step by step how to do it
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When we refer to this diagram, We can see correct terminal for the winding for each phase :
*CAUTION: Please refer to the name plate of motor to confirm the winding numbering ( U1,U2,V1,V2,W1,W2 ) and the motor connection of winding.
Why it very important??Because of each manufacturing have their own style for numbering and winding motor connection.
Star Delta phase and terminals
RED PHASE : U1 and W2 YELLOW PHASE : U2 and V1 BLUE PHASE : V2 and W1 So. From this formula, we must wiring the motor following the phase color code.
We can refer the complete wiring for star delta starter diagram below.If you want change their rotation for clock-wise or anti clock-wise,you need change two of phase ( RED or BLUE) at delta contactor.I share the technique how to change rotation in my next post.
If you want motor rotation for clock-wise,the phase colors are RED,YELLOW,BLUE.But if you want anti clock-wise rotation,the phase colors are BLUE,YELLOW,RED.For star delta control wiring diagram,please refer to my post about star delta diagram control
See example below for sizing the contactor.
If we have a 20hp motor (20746 watt=14920 watt), and it requires a star delta control system. What should be the three contactor size?
o First, you must know the Full Load ampere for your motor.
FLA = 14920/(4151.732*0.9)
FLA = 23 Ampere
So for MAIN contactor
= FLA / FLI
= 23 ampere
For DELTA contactor
= FLA x 0.58
= 23 x 0.58
= 13.4 @ 15 ampere
For STAR contactor
= FLA x 0.33
= 23 x 0.33
= 7.6 @ 8 ampere
Please used AC 3 rating for contactor..please refer my last post about magnetic contactor sizing
NEMA CODE: 430.22 (C)
Wye-Start, Delta-Run Motor. For a wye-start, delta-run connected motor, the selection of branch-circuit conductors on the line side of the controller shall be based on the motor full-load current. The selection of conductors between the controller and the motor shall be based on 58 percent of the motor full-load current.
Why multiply with 0.58?
The Star/Delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are smaller than the single contactor used in a Direct On Line starter as they are controlling winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line.
There are two contactors that are close during run, often referred to as the main contactor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third of the motor rating.
Why star delta become common starter of motor control?
It is because this starter have a simple circuit diagram, Low cost application and can drive for high horse power motor with low friction and can reduce starting torque with acceptable condition.
But this starter also tricky for beginner electrician and who not familiar with 6 wire electric motor. Actually it not too difficult if we have fully understanding the principle of star delta motor, starter sequence, circuit diagram and their operation.
For more detail please read my last post about star delta starter, star delta motor connection and many more topics about star delta in motor control categories.
If have any question or suggestion, please drop down your comment and we can share others circuit diagram design. One of my circuit diagram not too clear but i attached together with another circuit diagram for reference.
Example for star delta circuit diagram
How to setting the timer for star delta starter
Here I would like to share some information and basic technique how to get the approximate times for Star Delta Starter change over timer. I hope from this post, It’s can give some ideas or clear view how to setting the times for Star Delta Starter change over timer.
How many second we need to setting for changeover timer from STAR to DELTA sequence for Star Delta Starter? This question is very popular among electrician around the world when handling the Star Delta Starter for their application.
From my experience, honestly it’s quite difficult to get the accurate times for star delta changeover. But we can get the approximate times with several monitoring technique and measuring equipment to decide the best setting for change over timer of Star Delta Starter. From this post, I would like to share some information and basic technique how to get the approximate times for Star Delta Starter change over timer. I hope from this post, it’s can give some ideas or clear view how to setting the times for Star Delta Starter change over timer.
How to get a time for change over of Star Delta Starter?
The important point that we should know about Star Delta Starter is when in STAR sequence it must running with 80% of full load speed for motor before change over to DELTA sequence.
From this point we can use it to get the approximate time for change over timer. Basically we can use the two measuring method following:
- Clamp on meter / ampere meter
This method we can measure the ampere during STAR sequence. It’s can start from 0 (zero) until peak value of ampere and it will in flat reading because the voltage applied for star delta starter to the individual motor winding is therefore reduced by a factor of 1√3 = 0.58 this connection amounts to approximately 30% of the delta values.
Prepare the stop watch to measure the time from 0 (zero) ampere until peak value during STAR sequence.After that use the time that we measure to setting change over for the Star Delta Starter timer.We need minimum two person to perform this measuring and start stop the starter. - RPM meter
This measuring method using the RPM meter to measure the speed of motor in RPM (Running Per Minute).Get the information for Full speed of motor at the motor nameplate eg ; 1400 rpm.
We can measure the speed of motor when space is available or the pulley or shaft not covered such as air-compressor. From the basic point for Star Delta Starter is it must running 80% of speed of motor, we can measure the speed of motor (RPM) during STAR sequence.
Set the stop watch and measure the time from starting (0 rpm ) until it reach 80% of motor speed. Example if full motor speed is 1400 rpm, so for 80% full speed is around 1,120 rpm. So stop the watch and take the time reading.
That is the time for change over from STAR to DELTA sequence. Setting your timer with the time that we record and test run for full sequence than take a look if it any abnormal phenomena.
Conclusion about timer setting for Star Delta Starter
These two methods that I’d explained above is a basic measurement technique that we can use to determine the approximate time for change over of star delta starter.
It’s not 100% accurate but at lease we can get the approximate time for change over timer of Star Delta Starter. Normally the base line for timer setting is around 10 until 20 second.
We can use the try and error method to get the best time for change over from STAR to DELTA.
This is a starting method that reduces the starting current and starting torque. Star delta starter design normally consists of three contactors, an overload relay and a timer for setting the time in the star-position (starting position).
For the star delta starter, a motor must be in delta connected during a normal run and the main purpose is to be able to use star delta starter.
Star delta starter received the starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start.
Star delta starter only works when the application is light loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.
Description of Star Delta Starter Operation
For star delta starter,the basic function is to enable the motor to start and the motor windings are configured in a star formation to the supply voltage.
The voltage applied for star delta starter to the individual motor winding is therefore reduced by a factor of 1√3 = 0.58 this connection amounts to approximately 30% of the delta values. The starting current is reduced to one third of the direct starting current.
Due to the reduced starting torque, the star-delta-connection is suitable for drives with a high inertia mass but a resistance torque which is low or only increases with increased speed. It is preferably used for applications where the drive is only put under a load after run-up.
After motor run-up, in most cases an automatic timing relay controls the switch-over from star to delta. The run-up using star connection should last until the motor has reached the approximate operational speed.
so that after switching to delta, as little post acceleration as possible is required. Post-acceleration in delta connection will instigate high currents as seen with direct on-line starting.
The duration of start in star connection depends on the motor load. During delta connection, the full mains voltage is applied to the motor windings.
To enable a switch-over from star to delta, the six ends of the motor winding are connected onto terminals. The contactors of a star-delta starter switch over the windings accordingly.
How star delta starter works?
Starting in star, the main contactor connects the mains to winding endings U1, V1, W1.The star contactor shorts winding endings U2, V2, W2.
After successful run-up, the star contactor switches itself off and the delta contactor connects terminals U1/V2, V1/W2,W1/U2.
When changing from star to delta, we should pay attention to ensure it running in the correct phase sequence.Incorrect phase sequence can lead to very high current peaks during the cold switch-over pause, due to the easy torque reduction following re-start.
High peaks current can damage the motor windings and stress the control gear unnecessarily. The rotation of the motor has to be considered as well. If wrong rotation, we should change the rotation by switch the phase. I discuss detail about How to change star delta starter motor rotation in my separate post.
For star delta starter circuit diagram, wiring technique and motor base termination, Please read my post for star delta motor connection. Also for simple star delta control circuit wiring and types of star delta starter, please refer to my post for Electro mechanical star delta.
Electro-Mechanical star delta starter
Star Delta starter is the most widely used application especially for Air-compressor and HVAC system. This starter used for 6 or 12 wire for motor terminal depend on electric motor that chosen for the application.
Basically this starter used 3 unit contactor,1 unit timer delay,1 unit overload relay and start/stop button to control the star delta motor. Around the world, it have many types of control circuit design for star delta.
It have different design and technique for the control circuit but the main objective is same,to reduce starting inrush current.
For safety condition for the star delta control circuit, it include with interlock for star and delta sequence to avoid serious damage for electric motor. That mean during star sequence, contactor for delta cannot energize or vise versa.
This post i want share about two types typically used for star delta starter control circuit. It have a pro and contra for each design and it depend on our application and objective for installation.
Types of Star Delta control circuit
- Star Delta Open Transition
Firstly i explain why it call OPEN transition because the electrical motor is disconnected from main line when star sequence change to delta for momentarily condition. It include with interlock system for this control circuit.
For this open transition, the advantage is it depend on timing for transition when loading the electric motor. It cause the current surge and the high torque can effected electrical and mechanical due to shock on the application system.
It also can cause the instantaneous current peaks exceed the limit even the locked rotor current for short cycle of rotation. For the electrical aspect it can happen for the instantaneous peaks and could resulted the power fluctuations or losses.
Mechanical aspect it could cause the damage for system component such as drive shaft, bearing or coupling due to torque increased from current spike.
- Star Delta Close Transition
This CLOSE transition is an alternative type for star delta starter. The method is same for basic operation but it have additional a few devices to reduce and remove the surge in current matter that faced by OPEN transition type.
For this type, the design have the additional device and component such as a magnetic contactor and power resistors. The advantage of this method is it can eliminate the surge in current problem with high efficiency.
It can reduce a damage for motor winding and mechanical part. Theoretically it can make electric motor have a long life span and save cost for replacement part for mechanical items. For the clear comparison between OPEN and CLOSE transition, please refer to the graph below:
Comparison for OPEN and CLOSE transition Star Delta Starter
the science of Star Delta Motor Stater sourced from www.electricneutron.com hopefully with this knowledge useful for us lovers of motor stater.thanks.
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